''It was really spectacular,'' Dr. Hacker said in a telephone interview from her office at the Lamont-Doherty Geological Observatory in Palisades, N.Y. ''My voice transcripts (tape recordings taken during the deep-sea dive) became very unprofessional. I was screaming and yelling a lot and jumping up and down.''

It all happened earlier this month when the Woods Hole Oceanographic Institution research vessel Atlantis II headed out into the Gulf with a team of scientists who wanted to study what's known as the West Florida escarpment, the place where the continental shelf off Florida's coast suddenly drops to the Gulf floor.

The two scientists and pilot aboard the 25-foot-long research submarine Alvin were 10,500 feet below the surface of the Gulf about 135 miles west of the mouth of Tampa Bay, and they were approaching the base of the escarpment to study its erosion. The Alvin's search lights lit the Gulf floor in the pitch-black depths.

''As we moved toward the escarpment, we saw that the sediment turned a darker color, and we saw some large, dead mussels, much larger than there should have been at that depth,'' Dr. Hacker said.

Usually at that depth, she said, sea life is sparse, small, and limited to an occasional sea cucumber, since few nutrients on which animals feed drop that far down.

''Then we came across clumps of live mussels, and they were covered with tiny snails and limpids,'' she said. ''I was screaming, 'There's something going on here.' ''

What the scientists were looking at was something that had only been seen before in the eastern Pacific. In 1977, a research submarine found a similar community thousands of feet under the ocean, near where hot water laden with sulfur blasted through vents in the ocean floor.

That discovery overturned long-held biological assumptions that all life depended on photosynthesis - that life ultimately depended on energy from the sun, Dr. Hacker says.

Scientists at the time speculated that the creatures were feeding on bacteria that lived on the sulfur and heat coming from the vents, she said. If that were true, then no such communities should be in the Gulf, because no hot-water vents exist there. But here was a dense community of sea creatures living in total darkness without enough nutrients from the surface to support it.

''The density was compared to what you would find on a coral reef,'' she said , ''certainly not what you would expect to find at that depth.''

She said she saw tube worms 21/2 feet long, when they shouldn't have been more than three centimeters, and clams five or six inches across.

The Alvin's pilot, Ralph Hollis, had been on research dives in the eastern Pacific, says Dr. Hacker, and he identified these communities as the same type that surround the hot vents in the Pacific. But the water around the community was the same temperature as in the rest of the Gulf, and the scientists saw no places where water was being forced out of the the Gulf floor surface.

So how could the community survive? ''We could only assume there's a seepage of water with sulfur coming out of the base of the escarpment,'' Dr. Hacker says. ''There are a lot of sulfur springs in Florida, and it stands to reason that sulfur could be that far down.''

Charles Paull, a geologist with the Scripps Institution of Oceanography and leader of the expedition, agrees there is little doubt that somehow sulfur was seeping into the water there, probably in the form of hydrogen sulfide. But what the discovery shows is that heat is not a determining factor for the presence of this marine life.

Dr. Hacker says she expects to return to the Gulf to do more research on the communities.